The objective of this proposal is to measure the energy spectra of the three new neutron beams at M.D. Anderson Hospital, the University of Washington and the University of California at Los Angeles. This will be done using two techniques: (1) the foil activation technique in which neutron energy spectra are obtained by unfolding threshold foil activation data, and (2) the pulse height unfolding method in which both neutron and photon energy spectra are obtained by one-dimensional pulse height unfolding using experimentally measured response functions for mono-energetic photons and neutrons. Spectra will be measured in-air and in tissue-equivalent liquid phantom as a function of field size, depth along the central axis, off-axis distance, and beam modifying devices. Data measured under these conditions will be used to compute kerma ratios and ionization chamber sensitivity factors needed to calibrate the therapy beams. The results will also provide information on both the dosimetric and biological significance of the variations in the energy spectra within a phantom.